Multi-element geochemical modelling of crust-mantle interactions during late-Archaean crustal growth: the Closepet granite (South India)

The Closepet granite, in the Dharwar craton of south India, is a large, lat e Archaean magmatic body. Its composition can be explained as a result of i nteractions between mantle-derived melts and pre-existing continental crust (TTG gneisses). Using geochemical modelling based upon major and trace ele ment compositions the following petrogenetic model is proposed for the form ation of the batholith: (i) an enriched mantle (garnet- and amphibole-beari ng lherzolite) melts to produce a basaltic liquid. (ii) The basaltic liquid undergoes limited fractional crystallization of biotite and amphibole (F > 0.9). (iii) The differentiated liquid rises into the old continental crust , and induces water-saturated anatexis of the TTG gneisses (K-Feldspar + Qt z + Plagioclase melt). (iv) Both mantle-derived and anatectic liquid mix an d give rise to magma compositions ranging from quartz-monzonite to granite. The origin of the mantle enrichment is discussed. In the context of the re gional geological setting the most likely possibility seems to be metasomat ism by slab melts the metasomatism of a mantle wedge by slab melts. This su ggests a two-stage evolution for the Dharwar craton during the late Archaea n: (i) a subduction related event with the formation of TTGs, sanukitoids, and with associated mantle metasomatism; and (ii) re-melting of the metasom atized mantle. This evolutionary history implies that at least some of the K-rich, late-Archaean granites are juvenile, rather than products of intrac rustal reworking, as frequently assumed. (C) 2001 Elsevier Science B.V. All rights reserved.